The principle of microwave measurement of grain moisture and mass density has been very well described in technical papers produced by various researchers. Dr. S. O. Nelson and Dr. A. Kraszewski, USDA, Athens, GA. have provided numerous papers explaining the theory and experimental proof of the validity of using attenuation and phase wave parameters to determine the moisture content and mass of selected grains. The experimental work has emphasized the need to measure both attenuation and phase shift in the grain since both are density dependent functions. This work has led to using ratios of these two density dependent parameters to eliminate the density dependence.
Density-independent techniques are critical when grain is flowing through a system which contains a measurement in a free-fall state. Free-fall is thought to provide a convenient means to move the grain through the measurement system without mechanical flow control apparatus being placed within the moisture/mass sensor, but free-fall introduces significant additional problems as detailed in "Density-independent Microwave Measurement of Moisture content in Static and Flowing Grain", B. D. McLendon, B. G. Branch, S. A. Thompson, A. Kraszewski, S. O. Nelson, ASAE Vol. 36(3):827-835 - May-June 1993. Free-fall particles are not uniformly distributed throughout the confined space they are falling through. They actually cluster in changing geometrical patterns of varying densities. These clusters of particles then move around within the space because of the Magnus effect. In addition, the velocity of the falling particles varies in both the location of the particle relative to the wall of the confining space and between and within the clusters described above. When the grain falls it also accelerates through the sensor so the speed of the grain is increasing while the bulk density is decreasing in the vicinity of the RF measurement. All these variables are difficult to quantify and cause additional errors in the mass/moisture measurement.
It can be seen that if the attenuation measurement is to have meaning, the relative bulk density of the material being measured must simultaneously be determined. This measurement of the wave parameters, normalized to the bulk density, results in a meaningful measurement with the capability of providing an indication of the moisture content of the grain under test. Attempts to control the flow bulk density are discussed in the paper referenced above. A container of grain is placed above a controlled orifice and the grain is allowed to flow through the orifice in a slow controlled manner. This does reduce the increasing aeration of the grain while flowing, but it introduces another difficulty in that the grain is flowing in a small stream in the core of the container and another distribution in flow and bulk density occurs between the flowing grain core and the grain which is not flowing. In addition, this method is difficult to implement in the field while harvesting grain. The back-up of grain necessary before the flow restriction causes a significant decrease in the through-put of the harvester and presents a choke point or obstruction for the grain to jam in the machine sensor.
The copending application of George F. Nelson, Ser. No. 08/744,217, discloses a microwave grain monitor wherein grain is pushed upwardly, against gravity, through the grain tube of a harvesting machine by an auger, the grain tube extending through the microwave sensor region. This ensures that the bulk density of the grain is essentially constant, without a varying distribution of material bulk density through the sensor. Secondly, this ensures that the grain is fed into the sensor under positive flow conditions where all the grain can only move through the known cross-sectional area of the sensor. Under these conditions the wave parameters of phase and attenuation are not both needed to continuously measure the bulk density. We have determined that the effect of gravity and the grain particle shape causes the grain to always be packed to a density related to its moisture content hence measurement of the phase parameter is not required. Experiments in our laboratory have proven this to be true. The addition of the phase measurement increases the moisture and bulk density measurement accuracy only marginally, about 5.0% error versus 4.0% error.